This invention relates to a process for forming a resist pattern using exposure radiation having a wavelength below 180 nm, and a resist composition suited therefor.
In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) brought about a significant innovation, promising a possibility of commercial manufacture of devices on 0.18-micron rule. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, went through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated. With a wavelength reduction from KrF to ArF laser (193 nm), it is expected to enable miniaturization of the design rule to 0.13 xcexcm or less. Since conventionally used novolac resins and poly(vinyl phenol) resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 excimer laser (157 nm) which is expected to enable further miniaturization to 0.10 xcexcm or less, more difficulty arises in insuring transparency because it was found that acrylic resins are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. It was also found that poly(vinyl phenol) is somewhat improved in transmittance near 160 nm, but far below the practical level. It was found that reducing carbonyl and carbon-to-carbon double bonds is essential for insuring a transmittance. However, ring structures and carbon-to-carbon double bonds greatly contribute to an improvement in dry etching resistance. Polymers for ArF exposure in which benzene rings are excluded and instead, alicyclic structures are introduced for improving etching resistance are difficult to acquire transparency because their solubility resorts to carboxylic acid.
At present, reduction projection aligners having F2 laser mounted therein were developed and have just been marketed by Excitech in UK and Ultratech Stepper, Inc. in USA. However, they are not yet common, and under such circumstances, no frequent exposure experiments can be carried out. Then we analyzed the dissolution characteristics of resists upon exposure to F2 excimer laser light using a simulating illumination tool VUVES of Litho Tech Japan Corp. It was found that what becomes a problem as a result of wavelength reduction is a lowering of transparency, and in the case of a positive resist material, a negative working phenomenon that the exposed areas become insoluble as the dose of exposure is increased. Those portions which have turned negative are insoluble not only in alkali developers, but also in organic solvents such as acetone. This indicates that gel forms as a result of crosslinking of molecules together. Radical generation is presumably one cause of crosslinking. As a result of wavelength reduction, the exposure energy is increased so that even Cxe2x80x94C bonds and Cxe2x80x94H bonds may be excited in the case of F2 exposure (157 nm). As a result of excitation, radicals are generated with a possibility that molecules unite together. For polymers having an alicyclic structure for use in ArF exposure, for example, polynorbornene, an outstanding negative working phenomenon was observed. It is believed that these polymers have a structure susceptible to crosslinking since the alicyclic group has many Cxe2x80x94H bonds at the bridgehead. On the other hand, it is well known that xcex1-methylstyrene and derivatives thereof are effective for preventing crosslinking. Alpha-methylstyrene can mitigate the negative working phenomenon, but fail to completely eliminate the phenomenon. Moreover, since oxygen absorption is considerable in the VUV region, exposure is effected under the conditions that oxygen is purged, with an inert gas such as nitrogen or argon, to an oxygen concentration of 1 ppm or lower. Since oxygen is an effective radical trapping agent, this means that the radicals generated have a long lifetime and more crosslinking takes place. Among various resist polymers, an outstanding negative turning phenomenon was observed with resist materials using hydroxystyrene polymers as the base. In contrast, the negative turning phenomenon was scarcely observed with resist materials using acrylate polymers as the base. The resist materials based on acrylate polymers afford a fully high gamma value although they are non-fluorinated and low transparent. Then, we attempted to form a pattern by forming a mask on a MgF2 substrate, placing it in close contact with a resist film and irradiating by means of the tool VUVES. For the acrylate-based resist materials having a fully high y value, patterns with substantially perpendicular sidewalls were observed. It was thus confirmed that resists having a high contrast as demonstrated by a xcex3 in excess of 10 can form rectangular patterns although they are not so highly transparent.
It was found that alternating copolymers of a norbornene monomer having an acid labile group-bearing ester group directly bonded thereto with maleic anhydride are practically unacceptable in the event of F2 laser exposure, because they are very low in sensitivity and contrast (xcex3) as compared with the acrylic polymer resists.
In most cases, dry etching resistance is conventionally discussed in conjunction with the selection ratio of etching. As described in many reports, for example, J. Photopolymer Sci. and Technol., Vol. 5, No. 3 (1992), p. 439, J. Electrochem. Soc.: Solid-State Sci. and Technol., Vol. 130, No. 1, January 1983, p. 143, and SPIE, Vol. 2724, p. 365 (1996), engineers attempted to express the dry etching selectivity of a single layer resist using various parameters. Typical are Onishi parameter and ring parameter.
It was recently reported in SPIE, Vol. 3678, p. 1209 (1999) that micro-roughness develops on the resist surface after dry etching and is transferred after substrate processing and resist removal.
Making extensive studies, we found that the development of roughness after etching is observed when dry etching of SiO2 is carried out with a fluorocarbon gas such as CF4, CHF3, C2F6, C3F8 or C4F10 and that roughness increases under the high throughput conditions where the RF power is increased for high selectivity etching, that is, fast etching of oxide film. It was further found that roughness largely differs depending on the type of polymer used in ArF single layer resist. A noticeable roughness develops with acrylic polymers. In contrast, roughness declines with cycloolefin polymers such as norbornene homopolymers and alternating copolymers of norbornene with maleic anhydride. In particular, norbornene homopolymers give small values of roughness even compared with polyhydroxystyrene for KrF. Acrylic polymers with pendant adamantane exhibit a satisfactory value of etching speed, that is, selection ratio, fully comparable to cycloolefin polymers. When high selectivity etching was effected in an etching speed ratio of at least 3 between oxide film and resist, the surface roughness Rms of the etched surface as measured by atomic force microscope (AFM) was more than 15 nm for acrylic polymers and less than 3 nm for cycloolefin polymers. These results indicate that the selection ratio of etching does not necessarily coincide with the roughness after etching.
An object of the invention is to provide a resist composition which has a high transmittance to vacuum ultraviolet radiation below 180 nm, especially an F2 excimer laser beam (157 nm), Kr2 excimer laser beam (146 nm), KrAr excimer laser beam (134 nm) and Ar2 excimer laser beam (121 nm), as well as improved negative conversion-preventing effect, good dry etching resistance and minimized surface roughness after etching. Another object is to provide a patterning process using the same.
We have found that a polymer of an alicyclic hydrocarbon backbone such as norbornene, tricyclododecene or tetracyclododecene ring and having an acid-eliminatable ester group attached to the ring can be improved in acid elimination ability by introducing a spacer such as alkylene between the alicyclic hydrocarbon backbone and the acid-eliminatable ester group, and that using this polymer as a base resin, the above-discussed problems associated with exposure at a wavelength below 180 nm, especially F2 excimer laser can be overcome.
In one aspect, the invention provides a resist composition adapted for exposure to high-energy radiation having a wavelength below 180 nm, comprising (A) a base resin in the form of a polymer comprising recurring units having an alicyclic hydrocarbon backbone to which a carboxylate moiety capable of generating carboxylic acid when decomposed under acidic conditions is attached via 1 to 20 carbon atoms, (B) a photoacid generator, and (C) an organic solvent.
Preferably, the base resin (A) is a polymer of a norbornene derivative, tricyclododecene derivative or tetracyclododecene derivative.
Further preferably, the base resin (A) is a polymer comprising recurring units of the following formula (1), (2) or (3). 
Herein R1 is methylene, oxygen or sulfur, R2 is a straight, branched or cyclic alkylene group of 1 to 20 carbon atoms which may contain a hetero atom such as oxygen, nitrogen or sulfur to form a hydroxy, alkoxy, acetyl or ester group, R3 is an acid labile group, xe2x80x9caxe2x80x9d is 0 or 1, and xe2x80x9cbxe2x80x9d is 1 or 2.
In a preferred embodiment, the acid labile group represented by R3 in formulae (1) to (3) is a fluorinated acid labile group of the following general formula (4). 
Herein R4, R5, R6 and R7 each are hydrogen, fluorine, or a straight, branched or cyclic alkyl or fluorinated alkyl group of 1 to 20 carbon atoms, and at least one of R4, R5, R6 and R7 contains fluorine.
In a further preferred embodiment, the resist composition further includes a basic compound, a dissolution inhibitor and/or a dissolution improver.
In another aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the resist composition onto a substrate to form a coating; heat treating the coating and then exposing it to radiation having a wavelength below 180 nm through a photo mask; and optionally heat treating the exposed coating and developing it with a developer.